A 6-year-old boy presents with intense pruritus and thickened skin with accentuated skin markings in the antecubital and popliteal fossae. His mother reports a 3-year history of dry skin and recurrent flares triggered by soaps and stress. On examination, you note infraorbital creases and lateral eyebrow thinning. The structure marked **A** in the diagram shows the characteristic flexural lichenification pattern. Which of the following genetic mutations is most commonly associated with the pathogenesis of this condition?